1. Field of the Invention
The present invention relates to a lightweight cement panel, and more specifically to a lightweight cement panel which can be extensively used in the construction field, civil engineering field, etc., for example, as a panel for concrete formwork, or wall material, floor material, ceiling material, etc. of buildings, and furthermore, heat insulation material, etc. incorporated in partitions which divide a room.
2. Description of the Background Art
Conventionally, as concrete formwork panels, wood-based plywood (hereinafter simply called “plywood”) has been popularly used. However, plywood provides low durability and low water resistance, and has problems of absorbing water and increasing weight due to excess water at the time of casting concrete or rainfall at the time of storage, and at the same time, has a problem of degraded strength, and the number of repetitive use is about 3 to 7 times, which is small. Consequently, a large quantity of used plywood becomes construction waste, and it not only takes time and causes trouble to dispose of them by incineration, etc., but also results in degradation of global environment because a large quantity of lumber resources obtained from tropical forest are consumed by using plywoods. Therefore, it is requested to reduce the use of plywood. Furthermore, plywood is fabricated by laminating about six sheets of single panel and bonding them by polyvinyl acetate-based adhesives. Consequently, when plywood is disposed of by incineration, hazardous gas is generated from vinyl acetate of adhesives and causes environmental pollution. In addition, when plywood with vinyl acetate-based adhesives is applied for building residences, etc., sick-house problems occur, and plywood is unable to be used, in particular, for clean rooms, etc.
In concrete formwork panels, in view of the problems of plywood, various synthetic resin-made formwork panels are proposed (for example, see Patent Document 1 to 4). These synthetic resin-made panels do not consume lumber resources and are able to be repeatedly used 20 times to 30 times and furthermore have no problem of increased weight caused by water absorption or lowered strength. However, synthetic resin-made panels provide large variation in strength due to changes of ambient temperature, in particular, large variation in flexural rigidity as compared to plywood. Consequently, when concrete is cast in the summertime, the flexural rigidity of panel is degraded and the panel may be deformed due to cast concrete pressure. Furthermore, the synthetic resin-made panel may have a limited nailable range, depending on the construction, and it is difficult to say that the synthetic resin-made panel provides satisfactory processing capabilities as is the case with plywood.
In addition, as panels used for concrete casting, those made of cement-based, concrete-based, and other inorganic material are known. For example, permanent forms including cement-based composite material with glass fiber, unwoven cloth, etc. composed with glass fiber, etc. used for reinforcing material (see Patent Document 5), permanent forms including reinforced glass fiber mixed concrete plate (GRC) (see Patent Document 6), concrete structure using permanent form made with fiber-reinforced cement such as glass fiber, etc. (see Patent Document 7), cast-in form made with composite material fabricated by sandwiching and bonding glass fiber and other inorganic fiber mesh, stainless steel, iron, and other steel mesh, etc. between cement-based hardening material and (foamed) concrete-based hardening material (see Patent Document 8), permanent form for concrete casting fabricated by lightweight concrete board with cement paste covered in which on both surfaces of ready-mixed concrete board proper with lightweight aggregate and foaming agent mixed, netlike reinforced fiber sheet embedded and integrated (see Patent Document 9), etc. are known. However, these conventional panels for concrete casting including inorganic material are all permanent forms, which blend with cast concrete structure and compose part of the structure after concrete is cast, and are not repeatedly used as is the case with concrete formwork panels made with plywood.
In addition, many panels made with inorganic material using concrete or cement reinforcing fibers such as polyvinyl alcohol fibers (Vinylon), etc. are proposed. For example, a formwork panel for concrete pouring formed by a constituent containing gypsum and silica, glass fiber, water-reducing admixture and polyvinyl alcohol for alumina cement (see Patent Document 10), composite panel material having a fine base substrate layer composed with extruded hydraulic hardening material of hydraulic mixture including hydraulic inorganic material and reinforcing fiber material such as polyvinyl alcohol based fiber, polyolefin based fiber, polyamide based fiber, linen, glass fiber, carbon fiber, pulp fiber, and a porous packed bed laminated and coalescenced on one surface of the fine base substrate layer and having 0.6 or less specific gravity (see Patent Document 11), fiber-reinforced cement compact containing reinforcing fiber in which synthetic fiber A and polyvinyl alcohol based synthetic fiber B are mixed in such a manner that the weight ratio A/B of A and B achieves 0.1 to 3 (see Patent Document 12), fiber-reinforced board material based on hydraulic substance using meshed organization composed with spun yarn, such as polyvinyl alcohol based synthetic fiber as reinforcing material (see Patent Document 13), formwork that uses hydraulic paste containing at least hydraulic material, reinforcing fiber, and water and formwork at least part of the reinforcing fiber of which is polyvinyl alcohol based fiber (see Patent Document 14), hardboard containing fiber entwined material including polyvinyl alcohol based main-body fiber, in which the main-body fibers are bonded by polyvinyl alcohol based binder (see Patent Document 15), concrete structure having a structure main body formed by reinforcement rebar embedded and hardened concrete and fiber-reinforced high toughness cement board with cement which is integrated as permanent formwork onto the outer surface the structure main body used as active ingredients and high-strength Vinylon fiber used as reinforcing fiber (see Patent Document 16), precast concrete compact obtained by forming hydraulic constituent containing reinforcing fiber including polyvinyl alcohol based fiber and/or polyolefin-based fiber by centrifugal forming step (see Patent Document 17), fiber-reinforced exterior heat insulation material fabricated by laminating glass mesh or carbon mesh and urethane foam on one surface of high-strength high-toughness fiber-reinforced cement board formed by laminating glass mesh or carbon mesh and urethane foam on one surface of paper-made sheet in which reinforcing fiber contains polyvinyl alcohol based synthetic fiber (see Patent Document 18), etc. are proposed.
However, conventionally, there was no panel which provides durability that enables repetitive use equal to or higher than that of plywood and at the same time, provides processing capability, light weight, workability, handleability, etc. similar to those of plywood.    [Patent Document 1: Japanese Unexamined Patent Publication No. 6-66021    [Patent Document 2: Japanese Unexamined Patent Publication No. 8-207190    [Patent Document 3: Japanese Unexamined Patent Publication No. 11-182029    [Patent Document 4: Japanese Unexamined Patent Publication No. 2003-161037    [Patent Document 5: Japanese Unexamined Patent Publication No. 55-95728    [Patent Document 6: Japanese Examined Patent Publication No. 61-28511    [Patent Document 7: Japanese Unexamined Patent Publication No. 9-13486    [Patent Document 8: Japanese Unexamined Patent Publication No. 9-76404    [Patent Document 9: Japanese Unexamined Patent Publication No. 9-287239    [Patent Document 10: Japanese Unexamined Patent Publication No. 5-340090    [Patent Document 11: Japanese Unexamined Patent Publication No. 5-18042    [Patent Document 12: Japanese Unexamined Patent Publication No. 5-139803    [Patent Document 13: Japanese Unexamined Patent Publication No. 8-91911    [Patent Document 14: Japanese Unexamined Patent Publication No. 2001-252916    [Patent Document 15: Japanese Unexamined Patent Publication No. 2001-355171    [Patent Document 16: Japanese Unexamined Patent Publication No. 2004-36253    [Patent Document 17: Japanese Unexamined Patent Publication No. 2005-81657    [Patent Document 18: Japanese Unexamined Patent Publication No. 2006-112038